We are interested in the nuclear phosphoprotein p53 and of its possible role in cellular transformation to tumorigencity, and also in the normal cellular processes of differentiation and cell division. We have detected a specific 2kb p53 mRNA in chicken and also in fish cells. The p53 mRNA decreases drastically in the developing chicken embryo, similar to a decrease in the developing mouse embryo. These declines are apparently due to differences in the processing of the mRNA, and account for the decline of the p53 protein during embryonal differentiation. The complexing of the p53 with the SV40 T antigen in SV40 transformed cells is conditional; it depends on the type of cell studied, and it is unrelated to the transformation to the tumorigenic state. There is no correlation of the tumorigenic phenotype and of the steady state levels and of the half life of p53 protein and of mRNA, when well matched normal and derivative tumorigenic cells are compared. Our findings emphasize the postulated involvement of this evolutionarily well preserved protein in the general cellular processes of cell division and of differentiation, rather than in the tumorigenic transformation per se.